[R6RS] set-file-position! clarification

Although Windows does support Posix lseek and specifically allows the port
position to be set beyond the end of a file, the MSDN documentation does
not say whether the intervening bytes are zero filled if a write occurs at
the new position. Furthermore, the MSDN documentation for the Windows API
function SetFilePointer says that contents are unspecified in this case.
So I suggest that we follow suit and leave the unwritten intervening bytes
unspecified.
Kent